Multi-stage pulping process



United States Patent 3,145,135 P/IULTH=-STAGE PULPIING Julius ll). Robertson and Alfred M. Herald, Hartsrille, Silt, assignors to Sonoco Pruducts (Company Hartsville, Sail, a corporation of South Qarolina Fired dept. 4, 1962, Ser. No. 221,223 7 Ciahns. (till. Roz-43) This invention relates to a process for pulping wood chips and more particularly to a multi-stage pulping process and to the recovery of certain solubilized wood components from the wood chips pulped in such a process.

In the pulping of wood chips as generally practiced today, the wood chips are pulped in a digester in the presence of a cooking liquor or pulping chemical to convert certain wood components such as lignins into a solubilized form. These solubilized wood components are then removed from the wood chips and recovered by means of a leaching or washing operation. Although a single stage pulping process using a selected cooking liquor or pulping chemical is in widespread use today, it is also a common practice to subject wood chips to various cooking liquors or pulping chemicals in a plurality of cooking operations or stages carried out in sequence.

For instance, in multi-stage pulping processes, a first stage cooking liquor or pulping chemical is employed which is so compounded as to produce an ambient pH during digestion that may be either highly acid, slightly acid, neutral, alkaline or highly alkaline. The liquor or chemical of succeeding stages is usually so compounded as to produce a definite and significant shift or change in ambient pH from the first or previous stage. As is well known, the ambient pH of each stage determines in large measure the nature of the pulping reaction and the quality of the resulting pulp, since it controls such important factors as the lignin reaction, the rapidity of pulping reaction, the degree of attack on the hemicelluloses, the degradation of short chain non-crystalline fractions of cellulose, and the percent of alpha cellulose in the final pulp.

In the normal sequence of steps in a multi-stage pulping operation, a charge of wood chips is introduced into a digester and a first stage cooking liquor or pulping chemical is added. The charge of chips and cooking liquor or pulping chemical is then digested for a predetermined pediod of time at a selected temperature and pressure. The excess or free spent liquor from the first stage, containing certain inorganic material and solubilized wood components, is drained or blown from the digester and collected as desired. However, a considerable proportion of this first stage spent liquor is mechanically held in the chips so that it exerts a buffering or retarding action on the desired pH shift which the liquor or chemical of the succeeding stage is designed to bring about. Also, this retarding action may restrict the type of second stage pulping chemical which may be used. Furthermore, this first stage spent liquor held in the chips also contains dissolved organic matter which consumes chemical in the succeeding stage.

The second stage cooking liquor or pulping chemical is then introduced into the digester. After the buffering effect of the residual first stage spent liquor held in the chips has been overcome by the second stage liquor or chemical, the desired pH shift occurs followed by the second stage digestion of the chips. The above described pulp digestion and liquor removal operation may then be repeated for each succeeding stage of the multi-stage process. The spent liquor from all of the stages is collected either separately or together and the resulting pulp is washed with any suitable washing apparatus. The dilute filtrate or wash liquor is then added to the spent liquor collected from the various stages or to the separate spent Fatented Aug.

s in liquor of the last stage. The collected liquors are then concentrated, generally by evaporation, and utilized in various processes to recover their valuable constituents.

The undesirable action of the liquor mechanically held in the chips after the excess spent liquor is removed following each stage can be minimized by washing the chips in the digester at the conclusion of each stage. However, the recovery of the valuable constituents in the resulting filtrate involves a considerable expenditure of steam both for heating the wash water and subsequent evaporation of the wash liquor to effectively recover its valuable constituents.

In addition, it is also a characteristic feature of the usual multi-stage pulping practice, as the pulping digestion in each stage proceeds, that there is a progressive accumulation of the soluble products of the reaction between the pulping chemicals and certain components in the wood chips. The presence of these reaction products tends to waste chemicals and retard the desired pulping reaction and thereby extend the processing time for the production of desired pulping effects in each stage.

Accordingly, a primary object of this invention is to provide a new and novel multi-stage pulping process which permits efficient recovery of certain wood components and other valuable constituents from pulped chips and the most efficient utilization of the cooking liquors of the various stages.

Another object of this invention is to provide a new and novel multi-stage pulping process which permits thorough and inexpensive washing of the digested wood chips concomitantly with digestion in each pulping stage, utilizing only the Water present in the system, so as to obtain maximum effectiveness of the cooking liquor of each stage.

Still another object of this invention is to provide a new and novel multi-stage pulping process which permits the utilization of the water present in the system only as the cooking liquor for one or more of the pulping stages.

A further object of this invention is to provide a new and novel multi-stage pulping process in which the liquor mechanically held in the chips and the reaction products produced after each pulping stage are effectively removed so that the pulping process of the succeeding pulping stage can be carried out in a highly efficient manner with an attendant reduction in processing time and amount of pulping chemical required.

This invention further contemplates the provision of a new and novel multi-stage pulping process which provides for the recovery in a concentrated form of the valuable chemicals and certain wood components available from each digestion stage, which permits a selected type of cooking liquor to be employed in each digestion stage, and which enables the collection of all the material recovered from each stage in a common reservoir or in separate reservoirs, if desired, and at a concentration suitable for use without further evaporation in subsequent recovery processes.

Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawing.

In general, the objects of the invention and related objects are accomplished by digesting wood chips in a digester in the presence of a first stage cooking liquor or pulping chemical. This digestion is continued until the reaction between the wood lignins and cooking chemical is complete. The free or excess liquor is then removed from the digester and collected. The volatile components in this liquor are extracted as a relatively clear condensate and introduced into the digester for simultaneously delignifying and washing certain mechanically held solubilized wood components and other valuable constituents from the chips. This wash liquor is then combined with S)? the initially collected liquor. The wood chips are then subjected to a second stage pulping process utilizing a second stage cooking liquor or pulping chemical selected to bring about a definite shift in ambient pH in the digester from the ambient pH of the first stage digestion and thus predetermine the desired characteristics of the resulting pulp. During the second stage pulping process, the same liquor removing, condensate recovering and chips washing steps are carried out on the pulp and the resulting wash liquor is added to the collected spent second stage liquor for separate utilization or, as is preferred, the collected liquors of the second stage may be combined with the collected liquors of the first stage.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation may be best understood by reference to the following description taken in conjunction with accompanying drawing in Which:

FIGURE 1 is form of the apparatus for practicing the invention.

Referring now to the drawing, there is shown in FIG- URE 1 one form of apparatus suitable for carrying out the novel method of the invention. The novel method as described hereinafter is an improvement on the method described and claimed in the co-pending application of Julius D. Robertson and George M. Dickinson entitled Wood Pulping Process, Serial No. 199,748, filed June 4, 1962. It should be understood that the disclosure of the above co-pending patent application is incorporated within this application and reference thereto will be made here inafter only to the extent necessary to clearly define this invention.

Whereas the invention of the above co-pending patent application is directed to a method for carrying out a single stage pulping process on wood chips, this invention is directed to a multi-stage pulping process for wood chips which incorporates the novel features of the single-stage pulping process as described and claimed in the co-pending patent application. As is seen in the drawing, the apparatus for practicing the novel method of this invention is similar to the apparatus of the above co-pending patent application.

As shown in the drawing, the apparatus of the invention includes a digester designated generally by the numeral 6 having a side wall '7, a bottom wall 8 and a top wall 9 having an opening therein through which wood chips to be pulped are introduced. The digester 6 is closed by means of a lid or closure it which is clamped in the sealed position as shown by any suitable means to maintain the pressure in the interior of the digester.

A steam conduit 11 having a valve 12 therein, a pressure relief valve 13 and a cooking liquor conduit l4 communicate with the interior of the digester 6 through the side wall 7. Conduit l4 communicates with conduits is, 17 having valves 18, 19 therein respectively. Conduit l6 communicates with a supply of cooking liquor or pulping chemical for one pulping stage which supply i designated as Supply No. 1 and conduit 17 communicates with a supply of a second cooking liquor for another pulping stage which supply is designated as Supply No. 2. By way of illustration, Supply No. l is an alkaline cooking liquor and Supply No. 2 is a highly acid cooking liquor.

The bottom wall 8 of the digester 6 contains an outlet conduit 21 having a valve 22 therein and a conduit 23 communicating with conduits 24, 26 having valves 27, 23 therein respectively. Conduit 2.4 communicate with a liquor discharge conduit 29 having a valve 31 therein and a liquor storage conduit 32 having an outlet 33 extending within a pressure vessel 34 through the vessel top wall 36. The pressure vessel 34 is arranged to be filled with liquor 37 to a selected level indicated by the upper surface 38 of the liquor.

a diagrammatic view of the preferred the novel method of The inlet or suction side of a compressor 39 is connected by means of a conduit 41 to a zone 42 within the tank 34 between the liquor surface 38 and the vessel top wall 36. The outlet of the compressor 39 is connected by means of a conduit 43 to a coil 4 immersed in the liquor 3'7 in the tank The outlet end of the coil 44 is connected by means of a conduit 4d having a valve 47 therein to a conduit 43 communicating also with the conduit 26. The conduit 48 has a pump 49 positioned therein and communicates with a conduit 51 communicating with the inlet of a heat exchanger 52. A condensate bleed conduit 53 having valve 5'4 therein communicates with the conduit 51.

Heat is supplied to the heat exchanger 52 through a steam line 56 from a suitable source (not shown) and the steam condensate is removed by means of a conduit 57. The outlet of the heat exchanger 52 communicates with a conduit 58 communicating in turn with the interior of the digester 6 through its top wall 9.

In carrying out the novel method of the invention, a charge of Wood chips is introduced into the digester 6 and the lid 10 is clamped in the closed position as shown. Valves 12, 22 on the digester are opened and all other valves shown in the drawings, valves 13, 18, 19, 27, 28, 31, 47 and 54 are closed. Steam is then introduced into the top of the digester through the steam conduit ll from a suitable source (not shown) and valve 22 periits the escape of air and volatile material as well as the drainage of condensate accumulated in the digester through the conduit 21. The steaming is continued until the escape of a steady fiow of steam from the conduit 21 and/ or the attainment of a temperature of approximately C. in the digester which indicates that the flow of steam has tranversed the column of wood chips and completely filled the digester.

Valves 12, 22 are then closed and valve 18 in the conduit 16 is open to permit the introduction of a selected amount of first-stage cooking liquor from Supply No. 1 into the digester 6. In the illustrated embodiment, Supply No. 1 contains an alkaline cooking liquor and therefore the first stage digestion with this liquor constitutes an alkaline cook. As is well known, the cooking liquor employed determines the ambient pH in the digester which in turn determines the final characteristics of the resulting pulp. It should be understood that the cooking liquor employed in the first stage or in any succeeding stage is selected so as to produce a desired ambient pH in the digester for each stage which may be either highly acid, slightly acid, neutral, alkaline or highly alkaline. Furthermore, in practice the cooking liquor for each successive stage is selected so as to provide a distinct change the ambient pH from that of the previous stage.

Therefore, although the novel method of the invention is illustrated by an alkaline first stage cook and a highly acid second stage cook, the cooking liquor for each of these stages or any subsequent stages may be of any desired composition without department from the scope of the invention so as to produce the desired ambient pH of conditions for each stage.

Valve 18 is then closed, valve 28 opened and pump 49 is started to pump the liquor in the digester through the heat exchanger 52 which is now supplied with steam through the steam conduit 56. The digestion of the chips in the digester 6 is continued for about one hour at a pressure of approximately 100-110 p.s.i.g. and at a temperature of about l70 C. After this initial reaction and digestion period, the gases within the digester may be relieved through the valve 13.

Valve 28 is now closd and valves 27, 47 are opened so that the liquor is conducted out of the digester into the vessel or tank 34-. Compressor 39 is now started and the reduction of pressure on the surface 38 of the liquor 37 produces an evaporation of the volatile constituents (essentially water) in the liquor which are compressed in the compressor 39, discharged into coil 44 and condensed therein with the transfer of heat to the liquor 37. This relatively clear condensate is then pumped through the heat exchanger 52 by the pump 49 and is introduced into the digester so as to wash the mechanically held chemicals and solubilized wood components from the chips in the digester leaving chips free of such chemicals and reaction products which would inhibit the subsequent stage digestion process. This wash liquor then flows back into the vessel 34 in a continuous system constantly adding solids to the liquor 37 in the vessel.

As previously referred to, the cooking liquor used in each stage is selected so as to produce an ambient pH in the digester which is either highly acid, slightly acid, neutral, slightly alkaline or highly alkaline. In the illustrated embodiment, the alkaline first stage cooking liquor introduced into the digester may be any suitable, commonly used cooking liquor such as NaOH, Na CO Na SO or NaHSO or various combinations thereof. The first-stage cooking liquor may also be a highly acid cooking liquor such as NaHSO plus free H 80 the use of which is described hereinafter with reference to the second stage cook.

After the washing of the chips in the digester has been completed in the first stage and the liquors collected in the vessel 34, a pulping process similar to that of the first stage process may now be carried out on the Washed chips in the digester utilizing the second stage cooking liquor. To this end, valve 19 is opened to admit a selected amount of a second stage cooking liquor from Supply No. 2 through conduit 14 into the digester which liquor in this example, is highly acid. Valve 19 is then closed and the series of steps described above in the first stage process which include initial reaction and digestion liquor removing, condensate recovering and chip washing are performed on the wood chips with the resulting liquors preferably being combined with the previously collected liquors from the first stage. It should be understood that it is within the scope of the invention to remove all of the liquor resulting from the first stage pulping process from vessel 34 prior to beginning the second stage. Thus all of the liquors from the second stage pulping process can be similarly collected separately. Separate collection of the liquors from each stage is frequently desirable where the chemical characteristics of the various liquors differ so that a combination of the liquors would produce an undesirable mixture.

As has been explained, the cooking liquor or pulping chemical of the second stage is selected so as to produce a distinct change in the ambient pH in the digester. To that end, a highly acid cooking liquor is selected, in the illustrated embodiment, to produce this shift in ambient pH from the conditions in the digester resulting from the use of the alkaline cooking liquor of the first stage.

All of the liquors having been collected in the vessel 34 following the completion of the multi-stage pulping process of the invention, the opening of valve 31 in conduit 29 permits the collected liquor to be conducted to a suitable location for recovery of its valuable constituents. The pulped wood chips from which the solubilized wood components and other materials have been removed are blown through conduit 21 by opening valve 22 to permit utilization of the wood chips in a paper making process or the like.

In another form of the invention, it is possible under certain conditions to use only the water or condensate present in the system as the cooking liquor for one of the stages in the multi-stage pulping process. By way of example, if the first stage pulping process is a highly alkaline cook, the washing of the chips with the condensate extracted from the collector liquor reduces the highly alkaline ambient pH in the digester during the chip washing operation so that a desired shift in ambient pH is obtained providing a second stage pulping operation and resulting in what is in essence a two-stage pulping process.

Another outstanding feature of the invention is a modification of the process of the invention wherein cooking chemical for one or more of the pulping stages subsequent to the first pulping stage can be introduced into the digester during the washing of the chips carried out in the previous pulping stage. In carrying out this process, the condensate extracted from the collected liquor during one of the pulping stages such as the first stage pulping process is circulated through heat exchanger 52 and injected into the digester 6 as described above. As this condensate washing operation proceeds, the subsequent stage or second stage cooking chemical is introduced through conduit 53 into the condensate being pumped through conduit 51 so that a mixture of condensate and second stage cooking chemical is introduced into the digester 6 at a selected period during the washing operation and in a selected amount. This process has the effect of speeding up the multi-stage pulping process of the invention and produces the novel results accomplished as described above with the process of the invention.

It can be seen that the novel method of the invention results in an elimination of interference between stages in a multi-stage pulping process such as now results from the presence of reaction products, solubilized wood components and other material held in the chips after each pulping stage. By thoroughly washing the chips after each pulping stage without the addition of any wash Water other than what is present in the system, the cooking liquor or pulping chemical of succeeding stages is permitted to react effectively with the Wood chips without delay reducing the total pulping time in a multi-stage process and permitting maximum efiicient utilization of the cooking liquors or pulping chemicals employed. All of the liquor obtained from the various stages is collected in a concentrated form so that further evaporation is not required for utilization. The liquor resulting from each stage may be separately collected or collected together in a common vessel as desired. Furthermore, an even higher concentration of the collected liquors can be obtained by using the wash water or condensate in the system as a part of or substantially all of the vehicle for the cooking liquors or pulping chemicals of the pulping stages subsequent to the first stage.

While there has been described What at present is considered to be the preferred embodiment of the invention, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the invention and therefore it is the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Having thus described the invention, what is claimed is:

1. A multi-stage pulping process comprising the steps of, reacting wood chips in a digester in the presence of a first stage cooking liquor at a selected ambient pH by a sulfite pulping process to form solid lignosulfonates and partially complete the digestion of said. wood chips, removing the free liquor from said digester, recovering the volatile components in said liquor as a relatively clear condensate, introducing said condensate into said digester in intimate contact with said wod chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said wood chips while simultaneously removing by leaching said solubilized wood components from said Wood chips thereby forming a liquor, repeating said wood chips reacting step in the presence of a second stage cooking liquor selected to effect a shift in the ambient pH in the digester from the ambient pH of the first stage digestion and repeating said free liquor removing, condensate recovering and condensate introducing steps to complete said second stage digestion and to leach solubilized wood components from said wood chips thereby forming a liquor.

. 2. A multi-stage pulping process in accordance with claim 1 including the steps of, repeating said wood chips reacting, liquor removing, condensate recovering and condensate introducing steps in one or more additional stages subsequent to said second stage utilizing cooking liquors in each of said additional stages selected to produce a shift in the ambient pH in the digester between each stage.

3. A multi-stage pulping process in accordance with claim 1 wherein said first stage formed liquor is combined with said first stage free liquor and said second stage formed liquor is combined with said second stage free liquor.

4. A multi-stage pulping process in accordance with claim 3 wherein all of the liquors recovered from said pulping stages are combined in a common vessel.

5. A multi-stage pulping process comprising the steps of, reacting Wood chips in the digester by a sulfite pulping process in the presence of a first stage cooking liquor selected from a group consisting of a highly acid liquor, a slightly acid liquor, a neutral liquor, a slightly alkaline liquor and a highly alkaline liquor to form solid lignosulfonates and partially complete the digestion of said wood chips, removing the free liquor from said digester, recovering the volatile components in said liquor as a relatively clear condensate, introducing said condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said wood chips while simultaneously removing by leaching said solua. bilized wood components from said wood chips thereby forming a liquor, combining said first stage formed liquor with said free liquor, repeating said Wood chips reacting step in the presence of a second stage cooking liquor selected from said group to elfect a shift in the ambient pH in the digester from the ambient pH of the first stage digestion, repeating said free liquor removing, condensate recovering and condensate introducing steps to complete said second stage digestion and to leach solubilized wood components from said wood chips thereby forming a liquor, and adding the formed liquor of the second stage digestion to the second stage removed free liquor.

6. A multi-stage pulping process in accordance with claim 5 wherein said first stage cooking liquor is an alkaline liquor and wherein said second stage cooking liquor is an acid liquor. 1

7. A multi-stage pulping process in accordance with claim 5 wherein said first stage cooking liquor is an acid cooking liquor and wherein said second stage cooking liquor is an alkaline cooking liquor.

References Cited in the file of this patent UNITED ,STATES PATENTS 

1. A MULTI-STAGE PULPING PROCESS COMPRISING THE STEPS OF, REACTING WOOD CHIPS IN A DIGESTER IN THE PRESENCE OF A FIRST STAGE COOKING LIQUOR AT A SELECTED AMBIENT PH BY A SULFITE PULPING PROCESS TO FORM SOLID LIGNOSULFONATES AND PARTIALLY COMPLETE THE DIGESTION OF SAID WOOD CHIPS, REMOVING THE FREE LIQUOR FROM SAID DIGESTER, RECOVERING THE VOLATILE COMPONENTS IN SAID LIQUOR AS A RELATIVELY CLEAR CONDENSATE, INTRODUCING SAID CONDENSATE INTO SAID DIGESTER IN INTIMATE CONTACT WITH SAID WOOD CHIPS TO COMPLETE SAID DIGESTION BY HYDROLYTICALLY SOLUBILIZING SAID SOLID LIGNOSULFONATES AND OTHER COMPONENTS IN SAID WOOD CHIPS WHILE SIMULTANEOUSLY REMOVING BY LEACHING SAID SOLUBILIZED WOOD COMPONENTS FROM SAID WOOD CHIPS THEREBY FORMING A LIQUOR, REPEATING SAID WOOD CHIPS REACTING STEP IN THE PRESENCE OF A SECOND STAGE COOKING LIQUOR SELECTED TO EFFECT A SHIFT IN THE AMBIENT PH IN THE DIGESTER FROM THE AMBIENT PH OF THE FIRST STAGE DIGESTION AND REPEATING SAID FREE LIQUOR REMOVING, CONDENSATE RECOVERING AND CONDENSATE INTRODUCING STEPS TO COMPLETE SAID SECOND STAGE DIGESTION AND TO LEACH SOLUBILIZED WOOD COMPONENTS FROM SAID WOOD CHIPS THEREBY FORMING A LIQUOR. 